The present invention relates to print image enhancement techniques used in dot matrix printing machines, such as electrophotographic printing machines, and, more specifically, relates to matching standard digitized bit maps on a cell-by-cell basis (piece-wise) with prestored, predetermined patterns to provide compensating print signals to produce a synthetic, high resolution enhanced printed image.
Typically, non-impact printing machines are designed to create a printed image by printing a series of pictures elements, pixels or dots, on a print medium, such as paper. In electrophotographic printing machines, laser printers for example, a desired image may be created by a light source which is caused to scan across the charged surface of photosensitive material in a secession of scan lines. Each scan line is divided into pixel areas and the light or laser beam is modulated such that some pixel areas are exposed to light and some are not resulting in a predetermined series or pattern of overlapping pixels on each scan line. Wherever a pixel area is illuminated by the laser beam the photosensitive material is discharged. In that manner, the photosensitive material is caused to bear a charge pattern of pixels which images the subject that is being printed or reproduced. The printed copy is then obtained by developing the charged pattern and transferring the developed image to the print media.
The printed image produced by a dot matrix printer is a digitized or quantized image, sometimes referred to as a bit map image, of a desired analog image. An analog image cannot be exactly represented by a digital bit map image. The components of a analog image may be continuous in any orientation, while those of a bit map image must precede in orthogonal, incremental steps. This constraint results in distortion in the bit map representation of an analog image. The bit map image typically consists of a large number of discrete pixels or dots organized in a predetermined (spatial) pattern. Each dot has characteristics such as location coordinates, intensity, color and size. Each characteristic is independent of the other characteristics and can be viewed as an independent dimension.
The resolution of the bit map images produced by dot matrix printing machines is typically stated in the number of pixels or dots printed per inch. For example, a 300 dot per inch (dpi) printer has a higher resolution than a 240 dpi printer. A printer which produces 300 dpi in the horizonal row direction and 300 dpi in the vertical column direction has a 300 by 300 dpi resolution. At a resolution of 300 by 300 dpi lines printed parallel or perpendicular to the scan direction print with very little visible distortion. However, diagonal lines and boundaries between different regions of printer dots produce jagged steps or staircase distortion which is quite visible to the human eye.
Distortion in bit map representations is a consequence of low resolution of the bit map or low sampling rates of the desired analog image. A typical approach to reducing this distortion has been to increase the resolution of the bit map image by increasing the number of dots in a fixed size image, i.e., reducing the dot size, which increases the spatial resolution. Increasing the resolution reduces the size of the step distortions as well as preserving much fine detail which is lost at lower resolution. However, increasing the resolution is expensive. The amount of data to be processed and stored is proportional to the number of pixels or cells in the bit map. For example, doubling the resolution of a 300 by 300 dpi two-dimensional bit map results in a 600 by 600 dpi bit map which requires 4 times more memory and processing power. Further, a bit map image output device, cathode ray tube (CRT) or printer, for example, capable of displaying this higher resolution image must be used which may further increase the cost. If it is required to enhance the resolution on intensity level or colors as well, the cost will be further increased. While this solution has been used in many more sophisticated, high end printers, it is not a practical solution for lower cost, low end printers.
A second approach to reducing distortion caused by the digitization process utilizes interpolation techniques, to either link the outstanding corners of a jagged edge into a continuous slope or to average the intensity of neighboring dots and blur the jagged edge. While simple interpolation methods smooth the jagged or stairstep edges producing results generally more acceptable than straight forward bit map images, they have many side affects which are no less distracting than the original odd harmonic noise introduced by digitizing. For example, the gray scale anti-aliasing method blurs and smoothes the jagged edge transitions, but sacrifices contrast. The sharp features are averaged and smoothed, but many detailed features are also filtered out. In other words, the interpolation linking process causes additional data loss after the initial data loss due to the bit map conversion, digitalization, process.
U.S. Pat. No. 4,625,222 issued to Bassetti et al on Nov. 25, 1986 entitled, "Interacting Print Enhancement Techniques" discloses a technique for smoothing digitized staircase effects with a technique known as fine line broadening. To perform the smoothing function, dots of an intermediate visual level such as gray are produced along both edges of a slanted line adjacent the black dots which form the slanted line. For the broadening function, gray dots are added directly adjacent black dots in a dimension parallel to the scan direction while in a dimension perpendicular to the scan direction expanded black dots are produced. In combining the smoothing and broadening print enhancement techniques, various interactions are considered and enhanced dot producing signals are inhibited in some cases. U.S. Pat. No. 4,544,264 issued to Bassetti et al on Oct. 1, 1985 entitled, "Fine Line Print Enhancement" discloses techniques for enhancing the printing of fine lines in a direction parallel to the printer scan direction by providing a gray dot adjacent to the black dot on at least one side of the line. For fine lines in a direction perpendicular to the scan direction, the line is broadened by increasing the time period for dot modulation a predetermined amount greater than the normal dot. Thereby adding to the size of or broadening the line. Circuits are described which implement the print enhancement techniques and which are interposed between the character generator and the printer laser printhead. The print enhancement techniques disclosed in the above U.S. patents comprise line broadening techniques producing images in which the sharp edge features are not clearly defined with loss of fine detail.
U.S. Pat. No. 4,544,922 issued to Watanabe et al on Oct. 1, 1985 entitled, "Smoothing Circuit For Display Apparatus" discloses a smoothing technique comprising the selected addition or removal to or from particular portions of a dot pattern, of a small dot having a width 1/3 of the standard dot width. A smoothing circuit for a display apparatus is disclosed comprising memory means for memorizing data composed of selected standard width dots of a dot matrix representative of a character to be displayed and logical operation circuit means responsive to the data for performing logical operations thereon which satisfy predetermined conditions so as to selectively alter the data in correspondence with the addition or removal in the matrix position being considered of a small dot having a width 1/3 of the standard dot width. The selectively altered data presents the desired character with relatively smooth contours.
In more recent applications pattern recognition or template matching processes have been implemented. In these techniques template matching is implemented as a weighted matrix operator which is multiplied through every position of an arbitrary bit map image to provide a correlation of factors between the bit map image and the compared template or pattern. These complex matrix calculations can produce printed images with crisp contrast, clearly defined images and expanded dynamic range. Background noise (space invariant) can be selectively eliminated without loss of the detailed image features. However, this complex process is slow, hardware intensive and very costly which limits its applications to remotely sensed image processing used by the military or space research organizations.